Proper alignment of the meshing teeth of adjacent gears is important for the proper distribution of load through a gear train. Involute gear profiles are common in gear systems. As would be seen from a cross section of an involute gear tooth, the tooth is wider at the root and narrows along a specific curve until reaching a flattened tip. The region of the tooth along the specific curve is the face. The face of the tooth is an area from above the root to the tip, along the entire width of the tooth. The face area is the area that contacts the adjacent tooth, which has its own similar face area. The root end of the face of a driving tooth will initially contact/mesh with a tip end of a face of a driven tooth. If one were looking at a cross section of the mesh, this would appear to be at a point of contact, and the point of contact occurs at the moment the meshing gears engage. However, since this occurs along the entire width of the tooth, (from a first end to a second end), it is a line of contact, and the line of contact is formed when the meshing gears engage, and ends when the engagement of those meshing gears ends. As the driving tooth rotates, the line of contact will move from the base end of the driving tooth to the tip end of the driving tooth. The opposite is simultaneously happening on the driven tooth, where the line of contact is moving from the tip end of the face area to the root end of the face area. Once the line of contact of the driving tooth reaches the tip, the meshing teeth disengage and that line of contact ceases for those teeth. The next driving tooth takes over and the process repeats.
In an ideal environment, the axis of rotation of the driving gear and the axis of rotation of the driven gear are parallel, and separated by the proper distance. When the axes are parallel, a first end (along the width) of a driving tooth engages a first end of a driven tooth at the same moment a second end of the driving tooth engages a second end of the driven tooth. In other words, a first end of the line of contact and a second end of the line of contact engage at the same moment in time. It follows then that at the end of that mesh, the first end of the line of contact and the second end of the line of contact disengage at the same time. If the axis are not separated by the proper distance, the face area of driving and driven teeth changes. If the axes are too far apart, the teeth will have less face area to engage, and the face area and line of contact will move toward the tip of the teeth. This results in less engagement time to transfer the same torque, and more stress on the tooth because the engagement force has been moved more to the tip of the tooth. By their nature, involute teeth can handle a certain amount of excess distance, but any variation from “true” may increase stress and may reduce capacity or longevity of the component. True is defined herein as theoretically proper placement. If each component is true, then the entire system would also be true. If the axes are too close, the tip of one tooth will push too far into the region between meshing teeth. This may result in compressive forces on the one tooth, and a force to separate the meshing teeth, simply because there is no extra room between the meshing teeth.
If the axes of rotation are not parallel, i.e. they are angularly misaligned, the axial misalignment is likely to result in a situation where at one end the teeth wish to engage too much, and at the other end the teeth do not engage enough. In other words, the ends of the line of contact may not engage at the same time, and the line of contact may not be parallel to the axes of rotation of both gears. In such a case teeth may realize the consequences of both types of improper axial distances simultaneously, where there may be compressive forces on the first end, and reduced engagement at the second end of the line of engagement. As a result of this uneven distribution of torque, stresses on the teeth may again reduce capacity or longevity.
Situations where the axes of rotation are not the proper distance apart can result from simple manufacturing tolerances, where the bearing of one of the gears, which permits the gear to rotate, was manufactured/installed in a position other than the “true” position that would result in the proper distance apart. The same can be said of angular misalignment of the axes if, for example, the bearing was installed at an improper angle. Minimal misalignments can result in dramatic differences between an intended (i.e. “true”) load distribution, where there is no stress gradient through the line of contact because of proper meshing of gears, and the actual load distribution, which may have significant stress gradients, even when the products are manufactured according to specifications. Angular misalignment may also occur under torque load, where the carrier plate to which the planet is attached may urge a planet pin forward at the planet pin's base, but movement of the opposite end of the planet pin, to which the gear may be attached is resisted by downstream gears. This may result in a cantilever effect on the planet pin, where one end of the planet pin may lead the other, or trail the other if the load directions are reversed.
The above describes simple, two gear systems. Gear systems, however, are rarely that simple. Often, as is in the case of planetary gears, a gear may have more than one meshing/adjacent gear. In such cases, misalignments may have a cumulative effect, exacerbating the problem.
Stress gradients require manufacturers to design and build for the worst case scenario. As a result, manufactures have to design and build to the highest stress, although much of the system may not see these high levels of stress. For example, if stresses in the first end of the tooth are greater than the second end due to axial misalignment, the entire tooth must be designed and built to withstand the forces present only on the first end. This results in more expensive and heavier components, much of which is not properly utilized. Recognizing this, manufacturers have an interest in more uniform load distribution through the components. More uniform load distributions permit greater torque to be transmitted using the beefier components, or enable the manufacturer to build a leaner product that delivers the same torque as a product with greater stress gradients.
In an effort to reduce these stress gradients manufacturers have sought ways to reduce misalignment of meshing gears. One approach has been to mount one gear on a flexible shaft. Such an approach permits a gear that is mounted too close to another to be pushed away slightly, but that may result in axial misalignment of the meshing gears. Another approach involves a flexible planet pin mounted at one end, a gear mounted on the other end, and a weakened area in between. In this approach the planet pin flexes and the weakened area permits the gear to adjust, but in this approach the available adjustment of one end of the gear teeth is significantly different than the available adjustment of the other end of the gear teeth. Further, other mechanical configurations can be utilized. As such, there remains room for improvements in the art.